metamorphic
Banded Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (foliation); Luster: Dull to slightly vitreous; Structure: Gneissic banding with coarse grains; Cleavage: Generally poor, breaks along irregular surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating dark and light bands (foliation)
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (foliation); Luster: Dull to slightly vitreous; Structure: Gneissic banding with coarse grains; Cleavage: Generally poor, breaks along irregular surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure, causing minerals to recrystallize and segregate into bands. Often Precambrian in age.
Uses & applications
Used in construction as crushed stone, dimension stone for countertops and flooring, and occasionally as decorative landscaping pebbles.
Geological facts
Gneiss is one of the oldest rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct banding is called 'gneissic foliation'.
Field identification & locations
Identify by distinct light and dark mineral layers (typically quartz/feldspar and biotite/amphibole). Common in mountain ranges and glacial till. Collectors look for sharp, contrasting bands.
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